Why is the air let out of a tire valve stem so cold it will leave frost on the valve stem, plus why does the air used to operate air tools make them ice cold to the touch? Thank You

Question Date: 2007-02-03

Answer 1:

The temperature and pressure of a gas are directly related: when the pressure goes down, so does the temperature. Almost all bike or automobile tires are filled with pressurized air. A bike pump, for example, basically pushes more and more air into the tire. As you pump, the tube in the tire inflates and, once it's full, the volume of the tube doesn't change very much. If you keep pumping, you raise the pressure, squeezing more air molecules into the same space.

When you release air from the tire, the pressurized air comes shooting out and can expand. The pressure on the gas drops, lowering the temperature. CO2 cartridges are filled with pressurized CO2, so the gas shooting out of a punctured cartridge is also quite cold. Pneumatic air tools also work with pressurized air, wherever the pressure is let off the gas will cool as well. This property was used during the last World Cup to provide fast 'icing' for injured players, with doctors spraying the affected area with compressed gas.

If you want to learn more about this, search for "gas laws", specifically Avogadro's or the Ideal Gas Law.